Plasma jet devices, such as those of the direct current discharge type, which provide as ionized gas, are useful for generating excitation for spectrometric analysis or for studies of high-temperature chemical and physical phenomena of various materials.
U.S. Pat. No. 3,596,128, issued Jul. 27, 1971, describes an excitation source useful, for example, in spectroscopic analysis wherein a plasma jet device includes a swirl chamber surrounding an anode electrode and wherein a premixed atomized sample to be observed and an ionizing carrier gas is introduced into the chamber. In such plasma device and method, the plasma flame was directed through an orifice in the chamber, and the plasma flame bent to engage the cathode electrode at an angle to the axis of the plasma column. Although such a plasma device as described provided substantial improvement over prior art plasma devices, it has become apparent that there are certain difficulties associated with this device.
One of the difficulties involves the transport of the preatomized sample from the atomizing chamber to the swirl chamber surrounding the anode electrode. Other inherent problems in the use of a swirl-type chamber include the build up of aerosol particles within the chamber, and contamination of the exit port of the chamber with vapors of some of the elements to be observed; notably boron. In addition, studies have indicated that tangential introduction into the swirl chamber with subsequent uniform distribution of the sample surrounding the plasma through the exit port was highly inefficient in terms of the fraction of the sample to be observed actually reaching the region of maximum excitation of the sample; that is, the inner radius of the bend in the plasma column.
The patented plasma jet device discussed above, as well as a plasma device wherein the anode and cathode electrodes are positioned such that their axes, if extended, would intersect at an angle, has been discussed in an article "The Design and Some Emission Characteristics of an Economical dc Arc Plasmajet Excitation Source for Solution Analysis, " S. E. Valente and W. G. Schrenk, Applied Spectroscopy, Volume 24, Number 2, March - April 1970, pages 197- 205. This publication discusses the introduction of samples to be observed in a plasma jet device which comprises anode and cathode electrodes, each coaxially surrounded by a sleeve element through which an ionizing gas is introduced to form an inverted V-type ionized gas. Sample material to be observed in the excitation source was introduced coaxially through the anode sleeve. However, in such device and by such technique, the sample material contaminates the sleeve element through which it is introduced. Thus, as in the patented plasma device, such technique of introduction is highly inefficient in terms of the fraction of the sample which actually reaches the region of maximum excitation. Further, such method of introduction of the sample fails to achieve good penetration of the sample into the sheath surrounding the ionized gas, with the result that the emission intensity distribution of the sample is not wholly satisfactory. Accordingly, there is a need for a plasma jet device which overcomes the difficulties associated with such prior art devices and techniques of sample introduction.